The present invention relates to the adjustment of one or both of the narrow sides or side plates or wall plates in a mold for continuous casting of slab ingots and particularly for the casting of steel whereby adjustment is directed towards obtaining a particular conicity of and in the mold which conicity is needed to offset shrinking of the ingot on cooling; in addition the adjustability of the small sides is to accommodate molds of different rectangular cross sections.
Adjusting the sides in a mold for continuous casting generally is a complex task. The generally known practice in this regard involves in any event a particular load on the other skin of the casting having its major portion still in the liquidus state. Thus external load is effective particularly prior to the casting leaving the mold. It was found that widening of the mold increases the danger of skin rupture of the ingot or casting. This is so if the adjustment is to be carried out during the casting procedure and of course the adjusting speed is a major factor of concern. The latter aspect means that the adjusting speed has to be as low as feasible and any increase i.e. any more or less rapid widening of the mold does not appear to be practicable with the devices and methods used thus far.
In a symposium in Duisburg, West Germany, February 1984, Dr. Guenter Flemming read a paper entiled: "Format adjustment during casting--necessity, technology and engineering", reprinted on pages 121-143, published by Verein Deutscher Eisenhuettenleute/University of Duisburg, Germany. Herein the state of the art was summarized as follows. Past practice of mold changes is characterized by a period of approximately 10 years having been devoted to power and throughput increase, basically through increase of the speed of the continuous casting as such. Presently technology is directed towards more intensive cooling of the casting inside the mold in order to increase the sequence rate. Thus, present day efforts are devoted toward developing a casting program which is independent from any subsequent rolling program and permits long sequences. In conventional technology long sequences mean larger lot sizes, and that aspect leads to larger requirements for intermediate storage of castings in preparation for hot strip rolling.
On the other hand economic conditions are such that production costs generally have to be lowered meaning that intermediate storage prior to rolling should be kept as small as possible so that at least as far as practicality is concerned the casting process on one hand and the subsequent rolling process should not require extensive buffering. In addition, however, it should be realized that intermediate storage of continuous castings and ingots corresponds to the introduction of a cooling process while on the other hand and following a complete solidification of the casting the heat content thereof should be made available for subsequent hot rolling and to as large an extent as possible, particularly from the point of view of energy management.
Reducing intermediate storage and utilizing as much as possible the heat content of a casting presupposes that the ingot as produced by the casting machine has already exactly those dimensions needed for the subsequent rolling process. Thus, aside from the requirement for throughput increase of the casting machine by operation of long sequences one has to accommodate also the width dimensions of the rolling machine. This adaptation should occur during the casting process. Therefore it is the aim of production planning for continuous casting to tie the casting to the rolling program as much as possible so that rolling can indeed follow immediately the casting. Tying these two programs together means that within a short period of time considerable width changes in the casting have to be accommodated. This in turn implies that during casting at first a change to larger width as rapidly as possible should be accommodated following a reduction in width in more or less small steps until the program has been completed. These overall requirements pose therefore directly a specific task and problem towards adjusting the format in the casting machine. The critical aspect during adjusting the width of the mold is the support of the small or narrow sides. Adjustment of the narrow mold sides requires to some extent an "opening of the mold" so as to permit movement of the small sides in parallel. Accordingly a gap will appear during the adjustment between the mold content and a barely solidified skin on one hand and the mold wall on the other hand. Such a gap is immediately effective as a heat barrier i.e. the requisite heat transfer from the casting material into the mold wall particularly of the small sides is now impeded; at least as far as heat conduction is concerned; some cooling still occurs through thermal radiation but this heat transfer is insufficient. Moreover, the particular maximum gap is retained during the entire adjustment process.
It can readily be seen that insufficient cooling during the casting process establishes immediately and directly the danger of rupture of the skin. Particularly, if during the casting the small sides are moved outwardly towards an increase in ingot width these gaps form and pose the problem of rupture as mentioned. It is also apparent that the more pronounced the danger is, the higher the adjustment speed; the skin generally will be thinner than normal when the casting leaves the bottom of the mold.
Still referring to the state of the art, for several years in many parts of the world, modified programs of mold side movement have been investigated. In accordance with known technology the small sides are adjusted in three steps for purposes of increasing the ingot width. In accordance with the first step particularly in the beginning, the particular mold wall or walls are tilted about points in the lower part of the mold. In the second step a parallel displacement of the small side obtains i.e. the small side retains its orientation and the speed is matched to the speed of the casting. However, the paper referred to above does not explicitly explain the orders of magnitude involved in this matching procedure. In accordance with the third step, being so to speak a terminating step of the adjustment, a small mold side is tilted back in such a manner that in the upper portion the ingot, still being inside the mold, is slightly upset while in the lower portion of the mold a small gap between the mold wall and ingot or casting surface is deemed to be permissible. As far as this third step is concerned, it is not clear however how this slight upsetting of the ingot is supposed to occur in the upper part of the mold while in the lower part a gap is supposed to appear.
Finally the state of the art tends to optimize tilting in dependence upon casting speed and mold wall adjusting speed. In accordance with the above paper, nothing was said how optimized tilting in dependence upon these parameters is supposed to occur. It can thus be seen that the prior art can be summarized as follows.
Further increase of the mold wall adjusting speed does not seem to be justified since it seems to inevitably entail further loading and load exertion upon the skin of the casting which is simply not justified because of the rupture danger. Increasing the casting speed may reduce deformation as well as the gap but also means a larger transition between the two different dimensions of the casting which is the so called adjustment taper. Presently, adjusting speeds of about 15 mm/minute per side during increase and 20 mm/minute per side during reduction of the width of casting for a casting speed between 1.0 and 1.2 m/minute and a mold length of 700 mm are deemed to be the optimal values.
In order to drastically improve the state of the art, the mold length is a parameter which is of considerable importance. Standardized length for casting of slab ingots are for example 704 mm and 904 mm whereby it has to be noted that for such long molds usually one, two or several rollers are fastened to the lower ends of the small mold wall sides, possibly also on the wide sides and the active mold length has to consider the presence of these rollers. The so called short molds being used have a length of about 500 mm and they too include one, two or several rollers. Again the foot rollers have to be considered in considering the active mold length.
The equipment for adjusting one or several sides in a mold requires an optimized sequence of motions of machine parts and in accordance with the state of the art in order to obtain a high degree of flexibility as a whole. This means that the construction of the equipment for mold wall adjustment must permit independent motion for width adjustment as well as for adjusting the conicity. Construction of molds using copper plates for wide and narrow sides are generally known. The devices for adjusting one or two small side plates includes generally a pair of axially movable nuts being connected (linked) to the small mold sides. Usually driven threaded spindles are screwed into these nuts. The spindles of the pair can be driven at different speeds in order to obtain mold wall tilting. Both spindles will be driven from one motor via appropriate transmission gearing. Parallel shifting is also possible whereby however a change in conicity is not possible. On the other hand different pitches in the upper and lower spindles or different transmission ratios permit linear, width-dependent conicity changes. This kind of construction can be modified through using a coupling between the two spindles such as an electromagnetic clutch whereby in addition to the displacement of the narrow mold wall sides these clutches can be selectively deactivated in order to obtain a change in conicity. This means that optimized pivoting of the small mold wall side or sides about an upper as well as a lower part of the mold is actually not possible.
Another aspect of the state of the art is to be seen in the complete separation of the two spindles by providing separate drives for each of them. This kind of a design does indeed permit free adjustment for the mold wall sides for shifting and tilting. However, this independence in terms of structure requires a very high reliability with regard to electrically synchronising upper and lower drives because otherwise conicity may change in an uncontrollable fashion, at too high a rate; and even if brief severe ruptures may entail.
Thus summarizing this more accurate state of the art the previous conceptions in this regard can be described as follows. A speed controllable electromotor runs a gear for both adjusting spindles. Slight differences in speed reduction as applied to the different spindles establish the desired conicity over the entire range for the mold and mold width program. For each of the step widths such an adjustment is negligible small and does not have to be considered further. Moreover theoretically a very accurate parallel shifting of the small sides is possible.